Black spots in normal map
Your Subsurf modifier is set to the "Simple" algorithm, which in this case adds a lot of extra geometry without giving you any actual geometric smoothing (The normals are sampled more densely though which makes some difference in appearance). In any case you still have those strong edges in your geometry, that, combined with problably some form of floating point precision irregularities during raytracing give you these 'artifacts'. If you switch the SubSurf algorithm to "Catmull-Clark", remodel your tire pattern to make use of close proximity edge loops and the likes to retain the original shapes while gaining actual geometric smoothing via the SubSurf modifier, and then reattempt baking the normals, I would very much believe that your artifacts will be gone.

If anything (or all) of this sounds like gibberish, please let me know and i'll try to clarify further. :)